1.It is forbidden to inform upon another Jew to have him taken by non-Jews. Anyone who does so loses their share in Olam Haba. [1] Shulchan Aruch CM 388:9 Is it possible to do Teshuvah for informing a Jew to gentile authorities? If he is truly and sincerely sorry, is there a way to restore his Olam Haba? And if he informed the gentile authorities, but they didnt arrest the fellow Jew or do anything, then what would happen?

Shalom, It is possible to repent for any sin. The Rambam (Mishneh Torah, Hilchot Teshuvah 3:14 and 4:6) writes that even regarding the twenty-four sins that make repentance especially difficult, there is nevertheless no sin that prevents repentance. If a person sincerely repents, he is considered a ba'al teshuvah (one who has repented) and has a share in the World to Come. Rabbi Chaim Kanievsky (Kiryat Melech, ad loc.) cites the Rambam's sources, including: The Jerusalem Talmud: "There is nothing that stands in the way of repentance." And Tanna Devei Eliyahu Zuta: "Even if a person commits one hundred sins, one upon another, and then repents, the Holy One, blessed be He, says: 'I receive him with mercy and accept his repentance.' Even if a person spoke defiantly against Heaven and then repented, the Holy One, blessed be He, says: 'I forgive him for all his sins.'" However, for sins committed against another person, one must seek that person's forgiveness, as the Rambam writes (Hilchot Teshuvah 2:9): "Sins between a person and his fellow—such as injuring him, cursing him, robbing him, and the like—are never forgiven until he gives his fellow what he owes him and appeases him. Even after returning the money he owes, he must seek his forgiveness. Even if he only offended him verbally, he must appease him and continue asking for forgiveness until he is forgiven." In your case, you did not mention whether the other person suffered any actual harm. If no harm resulted, then there is no need to seek his forgiveness. One additional point is important to emphasize: There are cases in which it is not only permissible but even a mitzvah to report someone to the police. I have discussed this topic in detail here: https://www.yeshiva.org.il/ask/137018